Tensions between the two governments, which once enjoyed close relations, have spilled over into Lebanon's domestic politics, where the anti-Syrian ruling coalition remains locked in a 15-month power struggle against an opposition led by Shia Hezbollah, which is backed by Syria and Iran.
Lebanon has been without a president since Emile Lahoud, seen as pro-Syrian, ended his term in office on November 23, plunging the country into the worst political crisis since the end of the 1975-90 civil war.
Gordon Brown, the British prime minister, offered his Lebanese counterpart, Fouad Siniora, and the Arab League his full backing on Tuesday to help end an impasse over the election of a new president.
"His government has our strong support as it works to restore and strengthen the integrity of Lebanon's institutions, and to protect Lebanon's future as a tolerant and diverse democracy," Brown said.
Lebanon's parliament recently postponed a session to elect a new president until February 26, the 14th delay in three months.
Siniora on Tuesday warned that next month's Arab League summit would collapse if a solution to his country's political crisis was not found.
"I believe that the lack of representation of Lebanon on a presidential level at the summit will make the summit lose a lot of its importance," he told Reuters news agency.
"This summit should be attended by all the presidents and all the Arab countries."
Diplomats say King Abdullah of Saudi Arabia is unlikely to attend the Arab League's annual meeting unless Lebanon's political deadlock is resolved.
Riyadh has thrown its weight behind the Siniora government and mediated between Beirut and Damascus.